Frequency-programmed electron-capture detector

ABSTRACT

Improved circuitry for increasing the sensitivity of an electron-capture ionization detector includes a closed-loop feedback circuit which varies the frequency of pulses which are applied to the detector. The circuit responds to greater concentrations of predetermined compounds such as gases by increasing the pulse repetition frequency and responds to lower concentrations by decreasing the pulse repetition frequency, always tending to keep the current flowing in the detector circuit near a constant preset value. The pulse frequency will then vary directly with the concentration of sampled compound in the detector, and simple frequency-to-voltage conversion devices can be used to signal such concentrations.

United States Patent Marshall, III et al.

[ 51 June 20, 1972 [54] FREQUENCY-PROGRAMMED ELECTRON-CAPTURE DETECTORPrimary ExaminerJames W. Lawrence Assistant E.\'aminerD. C. Nelms [72]Inventors: J. Howard Marshall, in, Pasadena;

Timothy M. Harrington, Sierra Madre, Attorney-Leonard 00love et both ofCalif. ABSTRACT 73 Assi nee: Analo Technolo Co ration l g g gy rpoImproved circuitry for increasing the sensitivity of an elec- [22]Filed: March 15, 1971 tron-capture ionization detector includes aclosed-loop feed- [211 App! No: 124,291 baclt circuit which varies thefrequency of pulses which are applied to the detector. The circuitresponds to greater concentrations of predetermined compounds such asgases by inl -250/43-5 9 250/83-6 FT creasing the pulse repetitionfrequency and responds to lower [5 l] It'll. Cl. ..G0ln 23/12concentrations by decreasing the pulse repetition frequency [58] Fleldof Search R, 43.5 D, 43.5 FC, always tending to p the current flowing inthe detector 250/435 R cuit near a constant preset value. The pulsefrequency will References Cited then vary directly with theconcentration of sampled compound m the detector, and simplefrequency-to-voltage con- UNITED STATES PATENTS version devices can beused to signal such concentrations.

3,449,565 6/ l 969 Barringer ..250/43.5 R 10 Claims, 2 Drawing Figuresmzpw/z/m EAfl/Tfiflfff WWW f F [Jiffy/M676? Z5 5 0575670? 1 Z i I I,a/m/nm 4 ma 2 2 JHr) M 4 y] raw/mm; I 1 l/ W 0/52/ fi/ffifl/WP M5flin/rrm W V I 2: Fa m/[K v; k. T :1. [MW I -7-- i MUZZFA'TU? T 17] H,MMP 5mm) I iii/[6970B #5 i,- 4,.

u, [flflfif/l/T j:- iflff/M/Z T f; flfl/l/ff 14 T flI/UZH n if i r m/mmmm 2 Sheets-Sheet 1 Patented June 20, 1972 Patented June 20, 1972 2Sheets-Sheet 2 l l l l .J

I N VEN TOR 5.

rlllll II'I'II-lll'lll.

FREQUENCY-PROGRAMMED ELECTRON-CAPTURE DETECTOR BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to apparatusfor analyzing gases and vapors to determine the presence of compoundsand, more particularly, to apparatus for improving the range andsensitivity of electron-capture ionization detectors which are used insuch analyzing apparatus.

2. Description of the Prior Art In the copending application of ConradS. Josias, et al., Ser. No. 835,290, filed May 29, 1969, and assigned tothe assignee of the present invention, a gas detector and analyzer wasdescribed which utilized an electron-capture ionization detector tosignal the presence of very low concentrations of different chemicalcompounds in an environment. That application cited and relied upon aprior patent to James E. Lovelock, U. S. Pat. No. 3,247,375, whichtaught an electron-capture ionization detector and circuits which madesuch a device a useful tool for analysis.

Recently, Dr. James E. Lovelock delivered a paper entitled Analysis byGas Phase Electron Absorption at the Seventh International Symposium onGas Chromography Discussion Group of the Institute of Petroleum held atthe Falkoner Centret, Copenhagen, Denmark, from June 25 to June 28,l968. The paper was subsequently published by the Institute of Petroleumof London, W1, Great Britain in 1969 as part of a volume entitled GasChromotography, 1968," edited by C. I... A. Harbourn.

The Lovelock paper described in some detail the history of theelectron-capture detector noting that electron absorption was atechnique almost entirely dependent upon gas chromatography for itsexistence, the electron-capture detector being so sensitive that itcould function efficiently only with pure vapors greatly diluted in aclean stream of carrier gas emerging from a chromatograph column. Thatarticle is considered supplementary to and cumulative of Dr. Lovelock'sprior papers, including the article Ionization Methods for the Analysisof Gases and Vapors, published at page 162 in the Feb., I961, issue ofAnalytical Chemistry, Volume 33, No. 2, and a subsequent paper entitledElectron Absorption Detectors and the Technique for their Use inQuantitative and Qualitative Analysis by Gas Chromatography, publishedat page 474 of Analytical Chemistry, Volume 35, No. 4, of April l963.

In the Gas Chromatography 1968 article, Lovelock also described thechemical and physical basis for the operation of the electron-capturedetector and discussed the parameters that were important in theconstruction of such a detector. At page 102, Lovelock discussed themethods of operating such electron-capture detectors. A severe drawbackof the earliest versions was the limited dynamic range of suchdetectors. The DC method then employed applies a fixed potentialdifference between the electrodes of the detector. The detector issubjected to a stream of inert carrier gas which does not itself absorbelectrons. The potential is adjusted to a value sufficient to collectall of the electrons liberated from the carrier gas by a radiationsource which ionizes the gas.

An electron-absorbing vapor introduced into the gas stream collects thefree electrons to produce negative molecular ions which then recombinewith the positive ions resulting from ionizing radiation. The change incurrent flow attributable to the presence of electron-capturingcompounds is determined. If the decrease of current flow is measurable,then a quantitative indication of the electron-absorbing compound can beachieved.

Alternatively, the potential can be increased to a value that maintainsthe current flow at a constant value and the change of potential wouldalso represent a measure of the quantity of electron-absorbing compoundpresent. Yet other methods utilize higher potentials, but generally,such higher potentials result in a nonlinear response to vaporconcentration.

As described by Lovelock in the 1963 Analytical Chemistry paper, supra,a pulsed sampling technique can be employed involving the use of briefpulses of potential, at relatively infrequent intervals. Lovelocksuggested a 50-volt pulse of 0.5- microseconds duration, at intervals ofapproximately microseconds. This pulsed sampling procedure enjoyedseveral advantages in that:

1. For most of the time, no field is applied to the detector so thatfree electrons are in thermal equilibrium with gas molecules;

2. The sampling pulse is so brief that no significant movement ofnegative ions occurs, avoiding inaccuracies due to space-charge effectsor the collection of negative ions at the anode;

3. A pulse amplitude of 30 volts is sufficient to collect all of theelectrons;

4. The ultimate sensitivity is increased since the time for encounterbetween absorbing molecules and electrons is extended to the point wherenatural recombination limits any further increase in sensitivity; and

5. Except for those compounds whose absorption cross-sections increasegreatly with small increases in energy, and for which sensitivityimproves only in dc systems, the pulse method is much more reliable, andin general, sensitivity is improved.

In the copending Josias, et al. application, the pulsed samplingtechnique as described by Lovelock was modified. A highly-stable pulsesource, for example, a crystal-controlled oscillator whose frequencystability exceeds one part in 10 was provided. The magnitude of thepulses was reduced to approximately 30 volts, and the pulse duration wasextended to 3 microseconds. These pulses were repeated at 100-microsecond intervals. It appeared that the lower-voltage pulses oflonger duration also adequately swept all of the electrons from theionization detector and provided a current which, when averaged, couldbe used to signal concentration.

In the Gas Chromatography article, Lovelock, at pages 102 and 103,disclosed yet other improved pulse methods for increasing the dynamicrange of the detector. Detectors were described in which a signal formeasurement was not produced directly. Rather, the detector serves as asensor to indicate a departure from a steady-state condition.

One circuit was disclosed in which the output of an electrometeramplifier was fed back to a pulse generator where it was compared to areference current. The result of the comparison was used to set thepulse interval. The output of such a system was not a current to arecorder, but was a digital or frequency signal.

SUMMARY OF THE INVENTION Applicants herein have conceived of an improvedextendeddynamic-range device which they have tenned afrequencyprogrammed electron-capture detector." It is noted thatLovelock, at page 103 of the Gas Chromatography article, without theconsent of the inventors, discussed in general terms the presentinvention without describing it in detail. A block diagram was published(FIG. 6) which omitted some of the elements of the present inventionthat are deemed to be essential to the proper operation of theinvention.

According to the present invention, it is not enough to merely provide acomparator which compares the electrometer output with a reference tocontrol a pulse generator; it is also necessary that the reference beapplied in the form of a ramp signal.

The electrometer output is initially zeroed in the presence of a streamof pure carrier gas to establish a baseline. A reference voltage isapplied to a relaxation circuit, such that the voltage changes in alinear, ramp fashion between an upper and a lower reference voltage overan interval related to the period of the lowest useful frequency.

In one embodiment, the interval selected to provide maximum sensitivitywas 200 microseconds during which the ramp voltage had a lO-voltexcursion. The relationship between the electrometer output and theconcentration of a predetermined electron-capturing compound isextremely nonlinear. However, the relationship between frequency andconcentration is, for all practical purposes, a highly linear one. Thechange in frequency can then be a measure of concentration and candirectly provide an output signal.

The novel features which are believed to be characteristic of theinvention, both as to organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several preferred embodiments of theinvention are illustrated by way of example. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of afrequency-programmed electron-capture detector system; and

FIG. 2 is a circuit diagram of a preferred embodiment of the system ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning first to FIG. 1, thereis shown, in block diagram, a frequency-programmed electron-capturedetector system which is operable in a gas detector and analyzer such asthat disclosed in the copending application of Conrad S. Josias, et al.,Ser. No. 835,290, filed May 29, 1969, and assigned to the assignee ofthe present invention. There is shown a capturedetector circuit whichincludes the electron-capture detector 12. The capture detector 12 maybe identical to that disclosed in the above-identified Josias, et al.application or as described in any of the Lovelock publications. Theoutput of the capture detector 12 is applied to an electrometeramplifier 14, the output of which is fed back via an integrating circuit16 so that a steady-state output can be provided in response to a pulseinput. A baseline-adjusting circuit 18 also applies a current to theinput of the electrometer amplifier 14.

A comparator circuit 20 has one input connected to a mode switch 22which, in a first position, couples the comparator to a source ofreference voltage 24 and, in an alternative configuration, couples thecomparator 20 to the output of the electrometer amplifier 14. A secondinput to the comparator 20 is provided from a ramp generator 26, to bedescribed in greater detail below. The output of the comparator 20 isapplied to a pulse generator 28. It is the output of the pulse generator28 that represents the capture-detector circuit 10 signal output. Foroutput or display purposes, the pulse generator is coupled to afrequency-to-voltage converter 30 so that the concentration of theunknown electron-capturing compound can be displayed utilizing aconventional voltage meter or recorder (not shown).

The output of the pulse generator 28 is also applied to the rampgenerator 26, where it is used to initiate an interval dur ing which avoltage having a first value linearally changes to a second value at apredetermined rate. The output of the ramp generator, as noted above,provides the second input to the comparator 20. It is the comparison ofthe magnitude of the voltage of the ramp generator 26 with the magnitudeof the output of the electrometer amplifier 14 that, when equal,provides an output to the pulse generator 28 which, in turn, triggersthe generation of a pulse signal that initiates a new ramp interval inthe ramp generator 26.

The output of the pulse generator 28 is also applied to a switch 32which is coupled through a capacitor 34 to the detector 12.

The individual pulses generated by the pulse generator 28 and applied tothe switch 32 sweep the detector 12 of all charged particles and, asdescribed in the Josias, et al. application, provide an input to theelectrometer amplifier 14.

The pulses are also applied to an attenuator circuit 36 to provide adirect pulse output. Such an output can drive a digital counter (notshown), which can provide a number proportional to the concentration ofan electron-capturing compound.

The mode switch 22 when connected to the source of reference voltage 24(usually zero volts) causes the pulse generator 28 to operate at a fixedfrequency (usually 5 kHz) and enables a zeroing of the electrometeramplifier by appropriate adjustment of the baseline current through thebaseline-adjust circuit 18. This zeroing is done in the presence of apure carrier gas and establishes a lower signal limit which can beutilized to zero any display devices that may be used.

In the presence of an electron-capturing compound, the electrometeramplifier will provide an output signal to the comparator with the modeswitch 22 in the operating configuration. When the output of the rampgenerator 26 falls to a value equal to the voltage applied to thecomparator 20 through the mode switch 22, the pulse generator 28 will betriggered. As the magnitude of the output signal of the electrometeramplifier 14 increases resulting from a decrease in detector current,the equality relationship will be reached after an increasingly shorterinterval, and the pulse generator 28 will provide pulses at a higherfrequency.

The higher frequency of pulses applied to the detector 12 necessarilyreduces the probability that an electron will be captured by anelectronegative compound or a positive ion, thereby increasing thecurrent to the electrometer amplifier 14. The increased current outputthen requires that a longer portion of the ramp interval ensue beforeequality is reached, thereby reducing the frequency.

It will be seen that the system will stabilize at a frequency valuewhich is directly proportional to the concentration ofelectron-capturing compounds in the stream of carrier gas.

Given the apparatus illustrated in FIG. 1, the mathematical basis forthe linear relationship between frequency and concentration can bederived from a consideration of the following relationships.

Consider a mixture of a carrier gas 1 and an electron absorber compound2.

TABLE I t= time from the end of the application of a voltage pulse R,rate of electron production from a radioactive [3- emitter includingelectron-multiplication processes in the carrier gas N total number ofmolecules in the detector N number of positively-ionized carrier gasmolecules in the detector at t 0 N number of electron-absorbing gasmolecules in the detector at t O N, I number of ionized carrier-gasmolecules that have captured one electron at a time t N (z) number ofelectron-absorbing gas molecules that have captured one electron at atime t N,,(t) total number of electrons in the detector at a time I a,probability per unit time of capture of an electron by an ionizedcarrier-gas molecule which has not captured one electron (i.e.,recombination rate) 0 probability per unit time of capture of anelectron by an electron-capturing gas molecule which has not capturedone electron The rate of production of electrons is given by:

eU) e' dt production The first term in Equation (2) representsrecombination of electrons with positive ions in the carrier gas. Theseions were produced by the electron multiplicative processes startingwith the energetic a particles. The second term in Equation (2)represents the capture of electrons by electronegative gas molecules.Both of these terms can be trivially generalized to include more thantwo gas species.

Combining the differential Equations (1) and (2) gives the net rate ofelectron production:

was

Assume that at the end of a given period T all the electrons in thedetector are swept out by a voltage pulse with a width short withrespect to T. In addition, assume that N is negligible with respect toN,,. Then at equilibrium:

Integration and evaluation of the integration constant by utilization ofthe boundary condition N (t) when t 0 gives:

If a t l, the exponential may be expanded in a power series, neglectingall but the first, second and third order terms:

The average detector current is given by:

I N.,( )q

where T is the pulse interval and q is the charge on the electron (1.6 XC). In Equation (7) is contained implicitly the assumption thatEquations (1) and (2) apply during the period when the voltage pulse ispresent. This assumption is based on the discussion presented byWentworth, Chen and Lovelock on page 449 of their article appearing inthe Journal of Physical Chemistry, 70, 445 1966). In that discussion,they determined that, for a carrier gas containing 90 percent argon and10 percent methane, the average electron energy would be increased onlyslightly above its thermal value during the application of the pulse.They then presented data to confirm this prediction. From (6) and (7)the expression for the average detector current then becomes:

T or 2T2 1 Q If the baseline detector current is defined by 1B E q e (9)then the decrease in baseline current due to electron-capture processesis derived from (8) and (9):

For (alT)/3 l, the fractional decrease in baseline current is:

lrv l 1 2 (11 indicating that A! is approximately proportional to a,

If the definition of a, given in Equation (4) is used, Equation l 1)then becomes:

indicating the approximate proportionality of detector current change tothe number of electron-absorbing gas molecules in the detector.

In the derivation of Equation l l), a T/3 was assumed to be small withrespect to 1. Thus Equation 1 l) is accurate only if (2AI)/(3I is keptsmall, If the period Tis allowed to be programmed so that thisinequality holds, then the dynamic range for linear operation can bevastly increased over that resulting from fixed-period (orfixed-frequency) operation.

The relationship can be demonstrated by considering that the outputvoltage V of the electrometer 14 of FIG. 1 is proportional to thedecrease in detector current: 1,, 1,, A! (Equation 10).

In the preferred embodiment, the electrometer 14 was set for a maximumvalue of its output of +10 volts.

The voltage, V produced by the ramp generator 26 is an inverse saw-toothwave which is created by subtracting the voltage across a capacitorwithin the ramp generator from a fixed reference voltage, V,, which isset at +10 volts. The ramp generator 26 is designed so that thecapacitor discharges at a constant rate. Thus, the output of the rampgenerator at any time t is:

where 1, represents the current from a source within the ramp generatorand V V, at t 0. C, is the capacitance of the T 1Vs 2V. I.( AT) (14) Thepulse generator 28 also causes the switch 32 to apply a pulse to thedetector 12. This pulse sweeps the electrons from the detector in themanner described above and causes the detector current I to flow. Themagnitude of this current is described by Equations (7) and (5), and byEquation (11) when(2A1)/(3I,, l.

A current 1,, is subtracted from the detector current by thebaseline-adjust circuit 18. The difference of these currents then flowsin the electrometer 14 feedback resistor having a resistance R,. (Thefeedback capacitor having a capacitance C, filters the high-frequencyvariations of the detector current.) For an ideal electrometer 14, itsoutput voltage V becomes:

' 1 a o n) r Applying Equations 10), l4) and 15), one obtains for theperiod T:

If, in addition, it is assumed that 2Al/3I is negligible compared tounity, then Equation l l is valid and B-' O) r cnv. 1+ V. LAT I, C',a IR CrVr The frequency of oscillation f then becomes:

CrdlIBRf 1 1. 21. I.AT f 7,V, L (I I,)Rf C,Vr

Note that this frequency is approximately proportional to the number ofelectron-absorbing gas molecules in the detector N as can be seen fromEquations (4) and 18) combined.

in the preferred embodiment, the value of I., is chosen so that thefrequency f,,, resulting when N,,,=0, is made independent of R,. Thischoice allows the use of a relatively unstable resistor for R,, whichtypically has a value near ohms. ln

thiscase,

1 131 f ATVr IFIB 2f. :l R (1 and I. o.v.

that

In terms of concentration C, defined as the ratio of N ,to the totalnumber of molecules in the detector N then In order for this analysis tobe accurate, the inequality, 2A1 /3l,, l, must be satisfied. Themagnitude of this quantity can be calculated from Equations l 6) and(21) to yield:

The first term in this description of the deviation from Equation (1 l)is a constant as a function of N or f and thus does not represent anonlinearity in the relationship between f and N as described byEquation (18). The second term does represent such a nonlinearity, butcan be made infinitesimal by suitable choices of V, and R,. In thepreferred embodiment, V, +10 V, R,= 10 ohms, and I 3 X l0 A, so that 2A!I31, changes only :1 percent for f ZAI N Typically for a carrier gas inthe detector consisting of percent argon with 5 percent methane,

N a 1.5 kHz (26) For this case in the preferred embodiment, j, is chosento be 5 kHz so that the first term of Equation (25) has a value of 10percent.

The restriction that f, be large compared to is essential only forcomputational convenience in deriving Equation (17). Even if j, has alower frequency or the frequency corresponding to (5N is higher as aresult of electronegative gases being present, the proportionalitybetween Af and C still holds. The proportionality constant k must bemodified, however, to take into account such nonlinearities in thebaseline. Thus, one can write: Af E f f kC (27).

The lower limit for Af is given by the instabilities in the baselinefrequency fi In the preferred embodiment, these fluctuations aredetermined by small changes in the properties of the carrier gas (orimpurities contained in it) and not by electronic drifts. Typicallythese fluctuations are about 5 Hz for fi, 5 kHz in nearly pure 95percent A, 5 percent CI-l carrier gas.

The upper limit for f results from the non-zero time required to collectthe electrons from the detector. Typically this time is less than ns. Inthe preferred embodiment, an upper operating frequency of 5 MHz waschosen, resulting in a 200- ns minimum period between pulses.Experimentally it has been found that total charge collection can bemade to occur during the 50- to IOO-ns wide pulse from the switch 32. Aslight deviation from linear performance has been found at frequenciesabove 1 MHz, probably resulting from the fact that the electron energiesare not precisely thermal during the period that the pulse from switch32 is applied to the detector. However, satisfactorily linear operationhas been achieved using the preferred embodiment for values of Afbetween 5 Hz and 5 MHz, thus extending the linear dynamic range for theelectron-capture detector over six decades.

Therefore, it will be appreciated that the expected dynamic range of thedetector for compounds, such as sulfur hexafluoride (SP would beapproximately six decades. This dynamic range would also be applicableto electron-capturing compounds that are up to H100 as electronegativeas sulfur hexafluoride.

Turning next to FIG. 2, there is shown a preferred embodiment ofcircuits mechanizing the several blocks of FIG. 1. As illustrated, themode switch 22 is shown as alternatively connected to the referencesource 24 which, in the illustrated embodiment, is ground.Alternatively, the switch 22 connects to the output of the electrometeramplifier 14.

The comparator circuit 20 is mechanized by a pair of F ET devices 42a,42b. The electrometer signal is applied to the gate of one of the FETdevices 42a, and similarly, the reference signal from the ramp generatoris applied to the gate of the other of the FET device 42b.

The drains of the two FET devices 42a, 42b are coupled together througha pair of diodes 44, 46 connected in parallel in respectively oppositedirections. The output of the comparator 20 is provided from a pair ofcommon-emitter comparator transistors 48a, 4812, which are commonlycoupled through their emitters to a negative potential source 50.

In order to maintain the flow of current through the comparator, thesources of the FET devices 42a, 42b are commonly connected to thecollector of a supply transistor 52, the

emitter of which is connected to a source of positive potential 54through a resistor. The supply transistor 52 is operated as an amplifierto provide a reasonably constant current flow to the comparator 20. Avoltage divider 56, connected between the positive source 54 and acommon reference point 58, provides a predetermined bias to the base ofthe current-source transistor 52 to control the amount of currentsupplied thereby.

As long as the reference voltage exceeds the electrometer voltage, theFET device 42b conducts less than one-half of the current from thesupply transistor 52, while the FET device 42a conducts more than 50percent of this current. Similarly, the comparator transistor 48a isconducting, while the other comparator transistor 48b is heldnonconducting. The output of the conducting comparator transistor 48a isapplied to the pulse generator 28.

The voltage drop across the diodes 44, 46 is sufficient to create adifferential between the voltage applied to the base of the firstcomparator transistor 48a and the second comparator transistor 48b. Thisdifferential is sufficient to maintain the differential operation of thetransistor pair.

The pulse generator 28 includes a trigger transistor 60 and anaccelerating transistor 62 connected in parallel therewith. The outputof the trigger transistor 60 is applied to the base of first-stageinverter transistor 64, the output of which is applied to the base of asecond-stage inverter transistor 66, the output of which, in turn, isapplied in parallel to the bases of a pair of inverting outputtransistors 68 and 70.

The collector of the one output transistor 68 is coupled to thecollector of the other, complementing output transistor 70. The outputof the pulse generator 28 is taken from the common connection of thecollectors of the output transistors 68, 70. The output of the pulsegenerator 28 is applied to the switching circuit 32, afrequency-to-voltage conversion device 30, the ramp generator circuit26, and an attenuator 36.

The switch circuit 32 functions as a pulse amplifier operating in aswitching mode. An input transistor 72 applies its output to the base ofan intermediate-stage transistor 74, which in turn is coupled to thebase of an output-stage transistor 76. The input transistor 72 isnormally off, the intermediatestage transistor is normally on, and theoutput transistor is normally off."

The output transistor 76 is capacitively coupled as an emitter followerto the detector circuit 12. Further, the input to the base of the inputtransistor 72 is by way of a capacitive coupling so that the circuitresponds only to pulses, rather than to steady-state or DC levels.

The pulse output of the output transistor 68 of the pulse generator 28is also applied through a capacitive coupling to the emitter of afrequency-to-voltage transistor 78. The base of the frequency-to-voltagetransistor 78 is connected to the positive potential source 54, and thecollector is coupled through an RC filter circuit 81 to the source ofcommon potential 58. The values of the emitter-coupling capacitor 80 andthe capacitor in the filter circuit 81 are selected to provide a nearlysteady voltage output that is proportional to the frequency of theapplied input pulses within the operating frequency range. The timeconstant of the output filter circuit 81 determines the speed ofresponse of the analog output.

The pulse output of the pulse generator 28 is also applied to theramp-generator circuit 26, which includes a pair of transistors 82, 84connected together in parallel as normalmode choppers. A ramp capacitor86 is connected across the pair of transistors 82, 84 between a positiveprecision reference potential source 91 and a source of current suppliedby transistor 88a. During the positive portion of the pulse from thepulse generator 28, the ramp capacitor 86 is charged to a voltage V,,which is equal to the potential of the precision reference source 91.During the negative portion of the pulsegenerator pulse, the rampcapacitor 86 is permitted to decay linearly with time as a result of thecurrent supplied by transistor 88a.

The current source for the ramp generator is made up of a pair oftransistors 88a, 88b which are connected to a voltage divider 90.Transistors 88a and 88b are coupled and biased so that each branchcontributes an equal current flow through the common emitter resistor92, which is coupled to the negative potential source 50.

In operation, the ramp capacitor 86 is initially charged to thepotential of the positive precision reference potential source 91 by theaction of switching transistors 82, 84, which are held in conduction bythe output of the pulse generator 28 being near the positive potential58. The reference potential 91, which in the preferred embodiment is +10volts, is applied to the gate of the comparator FET device 42b, whichforces that device to conduct less than one-half of the current of thesupply transistor 52.

More than one-half of the supply current is drawn through the otherinput FET device 42a, which biases the comparator transistor 42a intoconduction. The bias of the pulse generator then forces the triggertransistor 60 out of conduction.

When the timing network 94 allows the accelerating transistor 62 to stopconducting, the intermediate-stage transistor 64 and the output-stagetransistor 68 are placed in conduction, and the other intermediate-stagetransistor 66 and the complementary output transistor 70 are forced outof conduction. The output of the pulse generator 28 then falls to thevoltage of the common reference point 58. Switching transistors 82, 84then stop conducting, and the voltage on the ramp capacitor 86 dropsaccording to the predetermined relationship. The voltage at the gate ofthe reference FET device 42b thus approaches the voltage which isapplied to the gate of the input FET device 42a.

The drain coupling diodes 42, 46 maintain a sufficient voltagedifferential between the bases of the comparator transistors 48a, 48b tomaintain the conduction of transistor 48a when the voltage on thereference gate exceeds the voltage on the input gate. As the voltage atthe reference gate approaches the input voltage, the reference FETdevice 42b begins to conduct more heavily and diverts current from theinput FET device 42a. The effect is reflected in the output ofcomparator transistor 480 which begins to turn off, resulting in avoltage rise at the base of the trigger transistor 60 of the pulsegenerator 28.

As the trigger transistor 60 begins to conduct, the intermediate-stagetransistor 64 and the output-stage transistor 68 are turned off," andthe second intermediate-stage transistor 66 and the complementing outputtransistor 70 begin to turn on. Considering the propagation delays, bythe time the complementary transistor 70 is turned on, the triggertransistor 60 is saturated in the conducting mode. The output of theoutput transistor 68 is sent back to a timing network 94 to turn on theaccelerating transistor 62 to maintain the intermediate-stage transistor64 in the nonconducting state.

The output of the intermediate-stage transistor 66 is coupled through adiode 96 to turn off the trigger transistor 60, thereby resetting it fora new pulse. However, the timing network 94 maintains the acceleratingtransistor 62 in conduction.

The leading edge of the output pulse produced by the output transistors68, 70 of the pulse generator 28 is applied to the ramp switchtransistors 82, 84, turning them on, which reapplies the full l0-voltpotential to the ramp capacitor 86. This results in the reapplication ofa 10-volt signal to the gate of the reference FET device 42b, therebyincreasing the conduction in the input FET device 42a and assuringconduction of transistor 48a. The resulting collector current oftransistor 48a then maintains the trigger transistor 60 of the pulsegenerator out of conduction until once again the reference voltage fallsto the value of the input voltage.

At this point, the output pulse has completed its rise and has reachedthe flat-top" portion that is maintained until the timing circuit 94decays sufficiently to turn off the accelerator transistor 62, therebyrestoring all of the other circuit elements to their original, quiescentstate. As the pulse at the output transistor 68 decays, the falling waveis applied to the base of the accelerating transistor 62 and rapidlydischarges the timing network 94 through diode 92 to restore the timingnetwork to its quiescent state in readiness for succeeding pulses, thusmaking the output pulse duration relatively independent of operatingfrequency. As the voltage of the ramp capacitor 86 again falls to avalue equal to the magnitude of the input signal from the electrometer,a new pulse will be generated and the ramp capacitor will again bereset.

In the preferred embodiment, the apparatus is calibrated by firstplacing the mode switch 22 in the zero position and adjusting theramp-current-source potentiometer 90 to produce a 5.0 kHz outputfrequency. This adjustment frequently will compensate for possibleerrors in the comparator circuit in determining the equality between theinput signal and the reference signal. The mode switch 22 is then placedin the "operate position, and the baseline adjustment 18 is varied togive an output frequency of .0 kHz while pure carrier gas is flowingthrough the electron-capture detector 12. This adjustment establishesthe conditions required by Equation 19).

To verify proper operation at the upper frequency end, the electrometersignal can be replaced with a voltage about 5 mV below the voltage ofthe lO-volt source. For this voltage the output frequency should be 5MHz for AT= lOO ms. Varying this frequency usually necessitates anadjustment of the response characteristics of the pulse-generatorcircuit 28, which, in this made, would be operating virtuallycontinuously.

Thus, there has been shown an improved frequency-programmedelectron-capture detector circuit. The improved circuit provides asignal whose frequency is proportional to the concentration ofelectron-capturing compounds in the sample under analysis. Moreover, theactual current and/or voltage of the electron-capture detector deviceand electrometer amplifier is not amplified to provide an analog-typeoutput signal, but rather is used to determine the output frequency.This output signal, in turn, controls the time available for exposure ofthe sample in the detector to thermalized electrons resulting fromradioactive decay.

As will be readily appreciated, higher concentrations ofelectron-capturing compounds will be subjected to relatively brieferintervals of exposure to the electrons, while low concentrations permitlonger exposure intervals. Accordingly, throughout the dynamic range ofthe apparatus, the number of electrons captured is small compared to thenumber produced, and thus the capture detector remains in a linearoperating region.

What is claimed as new is:

1. For use in combination with an electron-capture detector, apulse-generating circuit for providing an output signal having afrequency corresponding to and representative of the concentration of anelectroncapturing compound, the circuit comprising:

a. electrometer-amplifier means coupled to the electroncapture detectorfor providing a first output signal representative of the relativeconcentration of an electron-capturing compound in a sample quantity;

b. ramp-generator means, including a resettable oscillator circuitexhibiting a linear change-of-magnitude-with-time characteristic forproviding a second output signal of continuously varying magnitude;

c. comparator means coupled to said electrometer-amplifier means andsaid ramp-generator means and responsive to applied first and secondoutput signals, for providing a distinctive comparator output signalwhen said first and second output signals applied thereto are equal inmagnitude; and

d. pulse-generator means coupled to said comparator means,ramp-generator means and the electron-capture detector, saidpulse-generator means being operable in response to said distinctivecomparator output signals for applying a pulsed signal output to resetsaid ramp-generator means and to clear the electron-capture detector,

whereby said pulse generator produces a pulsed signal output trainhaving a frequency proportional to and representative of theconcentration of an electron-capturing compound in a sample.

2. The circuit of claim I, above, wherein said ramp-generator meansinclude in combination:

i. charging means adapted to store a predetermined quantum of electricalenergy;

ii. energy-source means coupled to said charging means for changing theelectrical energy stored therein at a predetermined, linear rate; and

iii. switching means connected to said pulse generator means and to saidcharging means for placing said charging means in a predetermined energystate in response to said pulse signal output.

3. The circuit of claim 2, above, wherein said charging means include acapacitor having one terminal connected to a first potential source;

i. said energy-source means include a current-source circuit coupled toa second potential source; and

ii. said switching means are connected across said capacitor for placingsaid capacitor in a predetermined state of charge.

4. The circuit of claim 1, above, wherein said comparator means include:

i. current-source means connected to a first potential source;

ii. a first branch connected between said current-source means and asecond potential source and being a control input coupled to saidramp-generator means, the current flow in said first branch beingcontrolled by the magnitude of said ramp-generator output; and

iii. a second branch connected between said current-source means and asecond potential source and having a control input coupled to saidelectrometer-amplifier means output, the current flow in said secondbranch being controlled by the magnitude of said electrometer-amplifiermeans output;

said second branch conducting virtually all of said current until themagnitude of said ramp generator output approaches equality with themagnitude of said electrometer-amplifier output, said ramp-generatoroutput, when reset, being of a magnitude sufficient to prevent currentflow in said first branch.

5. The circuit of claim 1, above, further including:

frequency-to-voltage converting means coupled to said pulse-generatormeans output for providing an output voltage signal proportional to andrepresentative of the frequency of said pulsed signal output train.

6. The circuit ofclaim 1, above, further including:

audible output means coupled to said pulse generator means forgenerating an audible output tone whose pitch corresponds to and isrepresentative of the frequency of said pulse signal output train.

7. The circuit of claim 1, above, further including modeswitching meansinterposed between said electrometer-amplifier means and said comparatormeans for applying in a first mode, the output of saidelectrometer-amplifier means to said comparator means and applying in asecond mode, a potential of predetermined magnitude for calibratingpurposes, whereby an upper frequency can be established for saidpulsegenerator means.

8. The circuit of claim 7, above, wherein a potential of approximately10 volts is applied to said comparator means to establish in saidcircuit a pulsed output train having a frequency of 5 MHz.

9. The circuit of claim 1, above, wherein said pulsed output signaltrain has a pulse repetition frequency of 5 MHz in response to anelectrometer-amplifier means input of substantially 10 volts and a pulserepetition frequency of 5 KHz in response to an electrometer-amplifiermeans input of substantially 0 volts, corresponding to an absence ofelectron-capturing compound in a sample quantity.

10. The circuit of claim 1, above, further including digital outputmeans coupled to said pulse generator means, including frequencyconverting means for dividing the frequency output of said pulsegenerator to a frequency range compatible with digital output deviceswhereby the digital output corresponds to and is representative of theconcentration of electron-capturing compound in a sample.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3.671.740Dated June 20 19 72 Inv entor(s) J. Howard Marshall, III and Timothy M.Harrington It is certified that error appears in they above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Col. 4, lines 73-74, Change Equation (2) to read as follows:

dN (t) dt N (t) [0 (N NA1) 0 N N removal Col. 5 line 4, After"energetic" change "a" to -B- line 12, Change Equation (3) toread asfollows an (a g Y R N (t) [q m N l) 0 (ND2 NA2) 1 line 5 6 After"Chemistry" change "7O" to '7 Q (add underscore) line 68, ChangeEquation (9) to read as follows:

I 5 qR Col. 6 lines 12-13, Change Equation (12) to read as follows T (N0 +N 0 line 63, After l" insert (closing parenthesis) Col. 7, line 74,the equation for I should read as follows:

IB 3 X 10' A Signed and sealed this 2nd day of January 1973.

/ (SEAL) Attest:

EDWARD M. FLETCHERJR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM powso USCOMM-DC 60376-P69 U45, GOVERNMENT PRINTINGOFFICE: I969 0-365-334

1. For use in combination with an electron-capture detector, apulse-generating circuit for providing an output signal having afrequency corresponding to and representative of the concentration of anelectron-capturing compound, the circuit comprising: a.electrometer-amplifier means coupled to the electron-capture detectorfor providing a first output signal representative of the relativeconcentration of an electron-capturing compound in a sample quantity; b.ramp-generator means, including a resettable oscillator circuitexhibiting a linear change-of-magnitude-with-time characteristic forproviding a second output signal of continuously varying magnitude; c.comparator means coupled to said electrometer-amplifier means and saidramp-generator means and responsive to applied first and second outputsignals, for providing a distinctive comparator output signal when saidfirst and second output signals applied thereto are equal in magnitude;and d. pulse-generator means coupled to said comparator means,rampgenerator means and the electron-capture detector, saidpulsegenerator means being operable in response to said distinctivecomparator output signals for applying a pulsed signal output to resetsaid ramp-generator means and to clear the electroncapture detector,whereby said pulse generator produces a pulsed signal output trainhaving a frequency proportional to and representative of theconcentration of an electron-capturing compound in a sample.
 2. Thecircuit of claim 1, above, wherein said ramp-generator means include incombination: i. charging means adapted to store a predetermined quantumof electrical energy; ii. energy-source means coupled to said chargingmeans for changing the electrical energy stored therein at apredetermined, linear rate; and iii. switching means connected to saidpulse generator means and to said charging means for placing saidcharging means in a predetermined energy state in response to said pulsesignal output.
 3. The circuit of claim 2, above, wherein said chargingmeans include a capacitor having one terminal connected to a firstpotential source; i. said energy-source means include a current-sourcecircuit coupled to a second potential source; and ii. said switchingmeans are connected across said capacitor for placing said capacitor ina predetermined state of charge.
 4. The circuit of claim 1, above,wherein said comparator means include: i. current-source means connectedto a first potential source; ii. a first branch connected between saidcurrent-source means and a second potential source and being a controlinput coupled to said ramp-generator means, the current flow in saidfirst branch being controlled by the magnitude of said ramp-generatoroutput; and iii. a second branch connected between said current-sourcemeans and a second potential source and having a control input coupledto said electrometer-amplifier means output, the current flow in saidsecond branch being controlled by the magnitude of saidelectrometer-amplifier means output; said second branch conductingvirtually all of said current until the magnitude of said ramp generatoroutput approaches equality with the magnitude of saidelectrometer-amplifier output, said ramp-generator output, when reset,being of a magnitude sufficient to prevent current flow in said firstbranch.
 5. The circuit of claim 1, above, further including:frequency-to-voltage converting means coupled to said pulse-generatormEans output for providing an output voltage signal proportional to andrepresentative of the frequency of said pulsed signal output train. 6.The circuit of claim 1, above, further including: audible output meanscoupled to said pulse generator means for generating an audible outputtone whose pitch corresponds to and is representative of the frequencyof said pulse signal output train.
 7. The circuit of claim 1, above,further including mode-switching means interposed between saidelectrometer-amplifier means and said comparator means for applying in afirst mode, the output of said electrometer-amplifier means to saidcomparator means and applying in a second mode, a potential ofpredetermined magnitude for calibrating purposes, whereby an upperfrequency can be established for said pulse-generator means.
 8. Thecircuit of claim 7, above, wherein a potential of approximately 10 voltsis applied to said comparator means to establish in said circuit apulsed output train having a frequency of 5 MHz.
 9. The circuit of claim1, above, wherein said pulsed output signal train has a pulse repetitionfrequency of 5 MHz in response to an electrometer-amplifier means inputof substantially 10 volts and a pulse repetition frequency of 5 KHz inresponse to an electrometer-amplifier means input of substantially 0volts, corresponding to an absence of electron-capturing compound in asample quantity.
 10. The circuit of claim 1, above, further includingdigital output means coupled to said pulse generator means, includingfrequency converting means for dividing the frequency output of saidpulse generator to a frequency range compatible with digital outputdevices whereby the digital output corresponds to and is representativeof the concentration of electron-capturing compound in a sample.